A non-volatile memory array includes non-volatile devices such as floating gate transistors. The floating gate transistors are programmed by storing a charge on the floating gate. The charge on the floating gate shifts the conductivity of the channel upon application of an activation voltage. The channel conductivity of the memory transistors (hence, the memory state) is detected by sensing the current flowing through the device.
The programming of memory cells is typically accomplished a word at a time but conventionally requires that the drain of selected cells be placed at e.g., six or seven volts, the gate at eleven or twelve volts, and the source at ground. This programming operation draws substantial source to drain current because the gate terminal is raised above the level of the drain and source terminals while a significant potential difference is placed between the drain and source terminals.
One way of improving the speed of the programming process is to program more devices in parallel. However, programming more transistors draws a larger current. The current to the transistors is provided by charge pump circuits due to the high voltage required for programming. Further, many elements of non volatile memories (such as gate dielectric) do not scale easily and limit any decrease in programming voltage.
Charge pump circuits provide a high voltage output from a lower voltage source. However, the maximum current that can be drawn from a charge pump is limited. An increase in total current output from the charge pump requires a corresponding increase in die area that is counter productive to the economics driving scaling.
Hence, what are needed in the art are improved circuits and ways of programming semiconductor memories.